A conventional thermal fixing device for an electro-photographic type image forming device includes a fixing belt, a heater such as a halogen lamp disposed at a space defined in an inner peripheral surface of the fixing belt, a nip plate and a pressure roller. A nip region is defined between the nip plate and the pressure roller through the fixing belt. A temperature sensor is provided to detect a temperature in order to control the heater for controlling a fixing temperature. More specifically, a recessed portion is formed at a surface of the nip plate, with which the fixing belt is in sliding contact, and the temperature sensor such as a contact type thermistor is disposed in the recessed portion.